1. Field of Invention
The present invention relates to a motor apparatus preferable for use in a flexible disk drive, having a structure of a frequency generator which outputs a FG signal for detecting a rotational velocity of a spindle, and particularly relates to a motor apparatus being compact in size and having high controllability and high noise resistance.
2. Description of Related Art
FIGS. 7(a) and 7(b) are perspective views of a motor apparatus having a frequency generator for detecting a rotational velocity of a spindle in accordance with the related art. Such the motor apparatus is used for a flexible disk drive having a spindle being a rotating shaft of the motor, which is so called a direct drive structure.
FIG. 7(a) shows a perspective view of a rotor portion of a motor apparatus. A circle shaped rotor 101 having a diameter xe2x80x9cDxe2x80x9d includes a FG magnet 102 placed on the outer circumference of the rotor 101. The FG magnet 102 is formed by mixing magnetic powder with nylon resin (so called xe2x80x9cplastic magnetxe2x80x9d) and having residual flux density xe2x80x9cBrxe2x80x9d of 0.18 T (Tesla).
FIG. 8 shows an enlarged view of a rotor shown in FIG. 7(a). FIG. 9 shows a side view of a motor apparatus. In FIG. 8, forty-eight (48) magnetic poles of FG magnetic pole 102a are provided on the rotor 101 in radial direction thereof. The diameter xe2x80x9cDxe2x80x9d of the rotor 101 in this case is 46 mm and a pitch xe2x80x9cWxe2x80x9d of the magnetic pole is 3 mm (xcfx80xc2x746/48=3).
FIG. 7(b) shows a stator of the motor apparatus. A stator 105 includes a stator base 103, a FG coil 104, a driving coil 108 and a shaft bearing 106. The FG coil 104 is placed on the circumference of the circle of which center is the shaft bearing 106. The stator base 103 is formed with a copper foiled printed circuit board through an insulative layer laminated over a base plate of soft magnetic material such as cold rolling iron and silicon steel. The FG coil 104 has a folded pattern with a folded pitch of 7.5xc2x0 (360/48=7.5) and opposing to the magnetic pole of the plastic magnet described above.
In FIGS. 7(a) and 7(b), a rotating shaft not shown is fixed in the center of the rotor 101, and is supported by the shaft bearing 106 at the center of the stator 105, and the FG magnet 102 opposes to the FG coil 104 with a gap xe2x80x9cGxe2x80x9d of 2 mm.
As the magnetic flux of the FG magnetic pole 102a shown in FIG. 8 interlinks to the FG coil 104, an FG signal is generated in the FG coil 104 by rotation of the rotor 104 in accordance with the Fleming""s rule. The frequency of the FG signal is in proportion of the rotational velocity. The structure described above constitutes a frequency generator.
The rotor 101 has disciform driving magnet (field magnet) 107 having sixteen (16) field magnetic poles, and the stator 105 has nine (9) driving coils 108 respectively. The rotational magnetic field occurs in the driving coil 107 by passing driving current through the driving circuit not shown, then the rotational force for the rotor 101 occurs by the interaction with field magnetic pole formed by the driving magnet 107.
The FG signal described above is converted to a voltage commensurate to the frequency by velocity controlling circuit not shown, and the converted voltage is given feedback to the driving circuit to control the rotational velocity of the motor constant.
However, the motor apparatus described above has following problems.
(1) The motor apparatus has problem of rotational control that the generated FG signal is very weak. Actually, the magnetic flux generated by the FG magnetic pole 102a reaches to the FG coil 104 for only 0.02 T. Accordingly, the magnetic flux leaked from the driving coil 108 adversely impact the magnetic flux of the FG coil 104.
The magnetic flux leaked from the driving coil 108 causes noise in the output of FG signal when it interlinks to the FG coil. Consequently, the velocity controlling circuit can not operate properly and the rotation of the motor fluctuates. Eventually, the rotation of the disk drive adversely impact the read-out/write-on operation of the flexible disk drive.
(2) The motor apparatus has disadvantage in miniaturization. The FG coil 104 and the driving coil 108 should be kept away from each other to suppress the impact of magnetic flux leaked from the driving coil 108, which causes the size of the motor apparatus bigger.
If the FG coil 104 and the driving coil 108 are kept away in horizontal direction, the diameter of the motor apparatus becomes bigger, and if the coils are kept away in vertical direction, the thickness of the motor apparatus becomes thicker.
In either case, the motor apparatus can not be miniaturized and the flexible disk drive having such the motor apparatus can not be miniaturized and the final product incorporating such the flexible disk drive can not be miniaturized.
(3) The FG coil is influenced by leaked magnetic flux coming from outside. The FG coil 104 is placed in the outer circumference of the motor which position is most sensitive for any leaked magnetic flux coming from outside. Leaking of magnetic flux can be blocked by magnetic shield. However, the magnetic shield is costly and needs more room for installation.
Accordingly, in consideration of the above-mentioned problems of the related art, an object of the present invention is to provide a motor apparatus a motor apparatus having a rotor placed rotatably on a stator base and provided with an FG signal for controlling the rotation of the rotor, the motor apparatus including, a driving magnet having an even number of magnetic poles evenly placed on the outer circumference of the rotor, wherein the even number is not less than 12 and not more than 32, and having a magnetic flux density pattern superposed with an xe2x80x9cnxe2x80x9dth (n is either one of 3 and 5) harmonic component, wherein the driving magnet is made of Ndxe2x80x94Fexe2x80x94B system material, and wherein a peak value of the magnetic flux density pattern of the driving magnet is in the range of 0.2 to 0.6 T (Tesla) in order to suppress rotational fluctuation caused by controlling the rotation of rotor within 5%, and an FG coil provided on the circumference of the stator base in the position opposing to each of the magnetic poles of the driving magnet with keeping a predetermined gap between the FG coil and the driving magnet, wherein the FG coil has a folding pattern of being folded alternately in the radial direction by a fold pitch angle of 1/n of a pitch angle of the driving magnet.
According to another aspect of the present invention, there provided a motor apparatus having a rotor placed rotatably on a stator base and provided with an FG signal for controlling the rotation of the rotor, the motor apparatus including, a driving magnet having an even number of magnetic poles evenly placed on the outer circumference of the rotor, wherein the even number is not less than 12 and not more than 32, and having a magnetic flux density pattern superposed with an xe2x80x9cnxe2x80x9dth (n is either one of 3 and 5) harmonic component, wherein the driving magnet is made of Ndxe2x80x94Fexe2x80x94B system material, and an FG coil provided on the circumference of the stator base in the position opposing to each of the magnetic poles of the driving magnet with keeping a predetermined gap between the FG coil and the driving magnet, wherein the FG coil has a folding pattern of being folded alternately in the radial direction by a fold pitch angle of 1/n of a pitch angle of the driving magnet, the motor apparatus further satisfying an equation of 4xe2x89xa6xcfx80D/(PG)xe2x89xa615 in order to suppress rotational fluctuation caused by controlling the rotation of rotor within 5%, wherein xcfx80 is circular constant, D is a diameter of the driving magnet, P is a number of magnetic pole of the driving magnet, and G is a gap length between the driving magnet and the FG coil.
Other object and further features of the present invention will be apparent from the following detailed description when lead-in conjunction with the accompanying drawings.